Quorum based transactionally consistent membership management in distributed storage

ABSTRACT

Systems and methods that restore a failed reconfiguration of nodes in a distributed environment. By analyzing reports from read/write quorums of nodes associated with a configuration, automatic recovery for data partitions can be facilitated. Moreover, a configuration manager component tracks current configurations for replication units and determines whether a reconfiguration is required (e.g., due to node failures, node recovery, replica additions/deletions, replica moves, or replica role changes, and the like.) Reconfigurations of data activated as being replicated from an old configuration to being replicated on a new configuration may be performed in a transactionally consistent manner based on dynamic quorums associated with the new configuration and the old configuration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of co-pending U.S. patent application Ser. No. 12/511,525 entitled “Quorum Based Transactionally Consistent Membership Management in Distributed Storage” and filed Jul. 29, 2009 which claims benefit under 35 U.S.C 119(e) of U.S. Provisional Application Ser. No. 61/107,938, filed on Oct. 23, 2008. The entireties of the aforementioned applications are incorporated herein by reference.

BACKGROUND

Advances in computer technology (e.g., microprocessor speed, memory capacity, data transfer bandwidth, software functionality, and the like) have generally contributed to increased computer application in various industries. Ever more powerful server systems, which are often configured as an array of servers, are commonly provided to service requests originating from external sources such as the World Wide Web, for example.

As the amount of available electronic data grows, it becomes more important to store such data in a manageable manner that facilitates user friendly and quick data searches and retrieval. Today, a common approach is to store electronic data in one or more databases. A typical database can be referred to as an organized collection of information with data structured such that a computer program can quickly search and select desired pieces of data, for example. Moreover, in such environments a federation refers to a group of organizations or service providers that have built trust among each other and enable sharing of user identity information amongst themselves.

With the advent of distributed computing models such as web services, there are increased interdependencies among entities such as a Service Providers (SP's.) Accordingly, a current trend is to focus on inter-organization and interdependent management of identity information rather than identity management solutions for internal use. Such can be referred to as federated identity management. In general, federated identity is a distributed computing construct that recognizes that individuals move between corporate boundaries at an increasingly frequent rate. Practical applications of federated identities are represented by large multinational companies that are required to manage several heterogeneous systems at the same time.

In such distributed systems, various challenges exist for proper management and configuration/reconfiguration of nodes. For example, individual nodes can fail randomly, which can cause data loss when suitable contingencies are not put into place. Likewise, replicated data is often required to be moved around the system, which can further create reliability issues and consistency problems.

Moreover, reliability issues can further complicate when data related to an over all management of such nodes are subject to loss due to failure of a centralized cache for example.

SUMMARY

Data in a transactional data store may be replicated across many computers or other devices acting as nodes in a distributed system, such as for redundancy or high availability purposes. However, while the distributed system may provide a high guarantee of availability, the underlying computers on which the transactional data store is replicated may themselves be unreliable, thus making it difficult to maintain transactional consistency of the transactional data store across the nodes in the distributed system.

A transactional data store configured to be replicated on each node of a set of nodes may be reconfigured to be replicated on each node of a different set of nodes.

The reconfiguration from a current configuration to a new configuration may be done in a transactionally consistent manner partially through the use of quorums. Each of the current and new configurations may be associated with at least one quorum, which may be less than or equal to the number of nodes in the configuration. Reconfiguration operations may be selectively performed such that they are only performed if a quorum of nodes in the configuration on which the operation will be performed indicate assent to the operation.

The reconfiguration process may involve a number of steps, including selecting a unique version identifier for the new configuration, updating the transactional data store replicated on nodes in the new configuration to a consistent state, deactivating the old configuration, and activating the current configuration. By requiring that progressing from one step to the following step of the reconfiguration process involves obtaining an acceptance or committal from a quorum of nodes in the configuration, it is possible to allow the reconfiguration process to be resilient to failures of nodes in a configuration, while still guaranteeing a level of consistency across the configuration.

The foregoing is a non-limiting summary of the invention, which is defined by the attached claims.

To the accomplishment of the foregoing and related ends, certain illustrative aspects of the claimed subject matter are described herein in connection with the following description and the annexed drawings. These aspects are indicative of various ways in which the subject matter may be practiced, all of which are intended to be within the scope of the claimed subject matter. Other advantages and novel features may become apparent from the following detailed description when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:

FIG. 1 illustrates a block diagram of a reconstruction component according to a particular aspect of the subject innovation;

FIG. 2 illustrates an exemplary partitioning and a rebuild associated with a plurality of nodes according to a particular aspect of the subject innovation;

FIG. 3 illustrates a block diagram of a system with a configuration component that can be reconstructed in accordance with an aspect of the subject innovation;

FIG. 4A illustrates a methodology of setting a new configuration according to an aspect of the subject innovation;

FIG. 4B is a block diagram of a network in which a reconfiguration process may be practiced;

FIG. 4C is a flowchart of a reconfiguration process;

FIG. 5 illustrates an exemplary environment for implementing various aspects of the subject innovation; and

FIG. 6 is a schematic block diagram of a sample computing environment that can be employed for data retrieval according to an aspect of the subject innovation.

DETAILED DESCRIPTION

The various aspects of the subject innovation are now described with reference to the annexed drawings, wherein like numerals refer to like or corresponding elements throughout. It should be understood, however, that the drawings and detailed description relating thereto are not intended to limit the claimed subject matter to the particular form disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed subject matter.

The inventors have appreciated that, with the advent of distributed computing systems, in which data may be replicated across many computers, a need exists for mechanisms to reconfigure replicated data in a transactionally consistent manner. The inventors have further appreciated that such distributed systems frequently have a need to offer high availability of the data, even as the underlying computers used to implement the distributed system may themselves occasionally crash. Transactional data, such as a portion of a database, may be replicated across a set of nodes, in which there may be a need to maintain transactional consistency for the data replicated across the set of nodes, even as nodes themselves are unreliable, and may therefore need to be added or deleted from the set.

Accordingly, in some embodiments of the invention, a current configuration including a set of nodes, each of which may contain a replica of a transactional data store, may be reconfigured in response to a reconfiguration request into a new configuration including a different set of nodes, all while maintaining transactional consistency across the replicated transactional data store.

The transactional consistency is also maintained throughout the reconfiguration process through the use of quorums, in which each of the old and new configurations may be associated with at least one quorum. Reconfiguration operations on the configuration may selectively proceed based on whether a quorum of nodes in the configuration indicate assent in the operation. The reconfiguration process may involve a number of steps, including selecting a unique version identifier for the new configuration, updating the transactional data store replicated on nodes in the new configuration to a consistent state, deactivating the old configuration, and activating the current configuration. Assuring that progressing from one step to the following step of the reconfiguration process requires obtaining an acceptance or committal from a quorum number of nodes in the configuration allows the reconfiguration process to be resilient to failures of nodes in a configuration, while still guaranteeing a level of consistency across the configuration.

Furthermore, other aspects of the system relate to reconstructing data/information related to reconfiguration of nodes, (the nodes are associated with a distributed system that implements dynamic quorums of read/write conditions) via a reconstruction component. In one aspect, the reconstruction component enables storing replicating partial copies of the information across the distributed system itself. Such distributed segments can then be employed to reconstruct content of the central management system in a consistent manner. Accordingly, the reconstruction component can reconstruct contents of a central management component from various locations on the system wherein the central management component/configuration component can be treated as a cache. Moreover, scalability can be provided via protocol partitioning of the central management component (e.g., employing a same protocol as employed to make other parts of the system highly available). Likewise, employing a central management component for leadership election for the rest of the system allows for flexibility and scale, (typically not afforded if using conventional consensus-based leadership election algorithm.)

FIG. 1 illustrates a block diagram for a system 100 that employs a reconstruction component 101, which enables reconstructing information related to reconfiguring members of a distributed system. Reconstruction component 101 can further be associated with a leader elector component 102 and a cluster configuration component 103, which can facilitate designation/operations associated with a primary (e.g., active) configuration manager instance/components.

FIG. 2 illustrates a block diagram for a system 200 that reconstructs a configuration manager component using a quorum-based approach according to an exemplary aspect.

FIG. 3 illustrates a block diagram of a system 300 that implements a configuration manager component 302 in conjunction with a plurality of nodes as part of a distributed environment such as a ring 310 which can be reconstructed in accordance with an aspect of the subject innovation. The configuration manager component 302 can reconfigure members of a distributed system of nodes (e.g., servers) form an old configuration to a new configuration, in a transactionally consistent manner by implementing dynamic quorums based read/write conditions, which mitigate data loss during such transformation. Such quorum can represent a predetermined number, wherein an addition of the read quorum and the write quorum exceeds number of nodes for the configuration (e.g., the read and write quorums of a given configuration overlap.)

FIG. 4A illustrates a related methodology 400 for various stages of configuring a network of nodes, which can be reconstructed in accordance with a further aspect of the subject innovation. Upon receipt of a reconfiguration request, the Reconfiguration Agent on the primary replica can initiate deactivation for an old or existing configuration, and supply a further activation of the new configuration (e.g., ensuring that any transactions whose commits were acknowledged to the client will be retained by the new configuration; and transactions which had not committed or whose commit had not been acknowledged can either be committed or rolled back). Such can include implementation of four stages, namely:

Phase 1: Ballot and Catch-Up at 402

During this phase the primary replica proposes a globally unique ID for the new configuration. The globally unique ID is intended to increase for each successive configuration. Upon acceptance by a quorum of replicas of both the old and new configurations, such an ID is guaranteed to be greater than any previously accepted ID for this partition. The proposed ID is sent to all replicas in both the old and new configurations each of which accepts or rejects the ID based on whether it is greater than any ID they have observed previously. Accordingly, if a replica accepts such ID it can further notify the primary replica of its latest transaction sequence number and halts acceptance of new transactions.

Alternatively, if a replica rejects the proposed ID, the primary picks a new higher ID and restarts Phase 1. Once a quorum of replicas from both the old and new configuration has accepted the proposed ID, the primary directs the replicas in the new configuration to start catching up so that the transactional consistency and data safety requirements are maintained across the reconfiguration. Such can involve a mixture of catch-up and transaction rollbacks on individual replicas. Moreover, the process is guaranteed to result in a quorum of replicas agreeing on the current state for the content and provides Atomicity, Consistency, Isolation, Durability (ACID) properties across the reconfiguration. Phase 1 can be complete once at least a quorum of replicas in the new configuration has been caught up.

Phase 2: Deactivation of Old Configuration at 404

During this phase the primary replica coordinates the deactivation of the old configuration. The purpose of deactivation is to guarantee that it is never possible to find two sets of replicas R1 and R2 such that R1≠R2 and each replica r1 in R1 claims that configuration C1 is the latest configuration and R1 forms at least a write quorum of C1 and each replica r2 in R2 claims that configuration C2 is the latest configuration and R2 forms at least a write quorum of C2; unless C1=C2. Moreover, a deactivation message can be sent to each replica in the old configuration. Each of the replicas can accept the deactivation if it matches the latest ballot proposal it has accepted. This phase is complete when a read quorum of replicas acknowledges the deactivation.

Phase 3: Activation of New Configuration at 406

During such phase the primary replica coordinates the activation of the new configuration. A purpose of activation is to guarantee that a write quorum of the new configuration knows that the configuration has been activated before changes to the content of the replication unit are allowed. Such can ensure that any content changes can be lost only if quorum of nodes are lost. The activation message can further be sent to each replica in the new configuration. Each of these replicas can accept the activation if it matches the latest ballot proposal it has accepted. Such phase is complete when a write quorum of replicas in the new configuration has accepted the activation. At this point the new configuration is active and useable.

Phase 4: Commit at 408

Such stage is an optional phase for committing the reconfiguration since by the end of Phase 3 the old configuration has been deactivated and the new configuration has been activated. Yet, such is known only to the primary replica and from a global outside-of-system perspective. Accordingly, such commit phase distributes this knowledge throughout all interested parties in the distributed system, namely to each replica in the old and new configurations. The Configuration Manager may be notified of the completion of reconfiguration at any point after the completion of Phase 3 regardless of whether Phase 4 is implemented.

FIGS. 4B and 4C provide more details of the reconfiguration process described in FIG. 4A above. FIG. 4B is an exemplary network 420 in which the reconfiguration process may be practiced. Network 420 includes a plurality of nodes communicating over any suitable computer communications media (e.g., wireless or wired communication, etc.), including a configuration manager 421, which may be similar to the configuration manager component discussed above in connection with FIGS. 1-3. Network 420 also includes a first set of nodes 422, 424 and 426 in a current (or “old”) configuration 428, and a second set of nodes 442, 444 and 446 in a new configuration 448. The first set of nodes and the second set of nodes may have any suitable relationship with respect to each other. For example, the first set of nodes and the second set of nodes may have one or more nodes in common, they may each comprise the same set of nodes, or the two sets of nodes may be completely disjoint. The plurality of nodes including the configuration manager 421, the nodes 422 . . . 426, and the nodes 442 . . . 446 may be implemented in any suitable way, including any suitable combination of hardware and software. For example, the configuration manager 421 and the nodes 422 . . . 426 may each be a desktop computer or server computer, using any suitable computer architecture, and may include an operating system, such as variants of the WINDOWS® operating system developed by Microsoft Corporation. In some embodiments, one or more nodes, for example, the configuration manager 421, may be implemented on a plurality of computers configured to operate as a coherent unit, such as for load balancing or redundancy purposes.

Each node may contain or be connected to storage media containing a replication of the distributed data. Each of nodes 422, 424 and 426 includes a replica of distributed data, illustrated as replicas 432, 434 and 436, respectively. The distributed data may be a component of a distributed storage system. For example, the distributed data replicated on nodes 422, 424 and 426 may be a portion of a distributed database, such as a data partition in the database. Any suitable distributed database may be used. In some embodiments, variants of the SQL Server® database developed by Microsoft Corporation may be used. Each of replicas 432, 434 and 436 may store a plurality of transactions, as discussed above.

A reconfiguration operation may change the manner in which data is stored. Rather than being replicated on a first set of nodes, illustrated in FIG. 4B as the current configuration 428, after a reconfiguration, the current information will be replicated on a second set of nodes, illustrated in FIG. 4B as the new configuration 448. Thus, at least by the end of the reconfiguration process, the nodes 442, 444 and 446 in the new configuration 428 may also each contain replicas 452, 454 and 456, respectively. The reconfiguration process may involve performing actual movement of data from the current configuration 428 to the new configuration 448, such as if a node in the new configuration 448 does not at first contain a replica of the data partition being replicated, or if the replica on a node in the new configuration 448 is out of date. However, in some embodiments or in some scenarios, the nodes in the new configuration may already contain an up-to-date replica of the data partition, in which case the reconfiguration process may primarily involve deactivation of the old configuration and activation of the new configuration.

In some embodiments, the reconfiguration process is initiated by the configuration manager 421. The configuration manager 421 may initiate a reconfiguration for a number of reasons. For example, reconfigurations can be triggered by a variety of events, include node or replica failure, node or replica recovery, replica creation, or other administrative actions. The reconfiguration process may be carried out on any suitable node in the network. In some embodiments, the configuration manager 421 may be primarily responsible for implementing the reconfiguration process, while in other embodiments, the configuration manager 421 may issue a reconfiguration request to another node in the network, which may then carry out the reconfiguration process in response to the request. In some embodiments, one node in the new configuration may be designated as a “primary node,” illustrated in FIG. 4B as “primary reconfiguration agent,” node 442, responsible for carrying out the reconfiguration process in response to a reconfiguration request from the configuration manager 428. Implementing the reconfiguration process may involve sending messages, such as reconfiguration messages 458 and 459 to and from the primary node 442 and other nodes in both the current configuration 428 and the new configuration 448.

FIG. 4C illustrates multiple ways in which quorum-based processing can be used in reconfiguration operations. FIG. 4C illustrates that as part of reconfiguring a distributed database, reconfiguration operations, such as deactivating the old configuration and activating the new configuration, are performed. If a quorum of nodes involved in each of these reconfiguration operations does not indicate acceptance of a message indicating that the reconfiguration operation is to be performed, the reconfiguration operation does not proceed. Conversely, when an appropriate quorum of the nodes involved in the reconfiguration operation indicates acceptance in response to a reconfiguration message or command, the reconfiguration operation may proceed.

As illustrated in FIG. 4C, a reconfiguration process may involve moving a data partition from a current (or “old”) configuration of nodes to a new configuration of nodes. While in the discussion that follows, the process illustrated in FIG. 4C is assumed to be implemented on the primary node in the new configuration and is assumed to start in response to receiving a reconfiguration request from a configuration manager, the process of FIG. 4C may be implemented on any suitable node in the network and may be started based on any suitable event, stimulus, or condition.

In some embodiments, each configuration of a data partition may be associated with a version identifier, so that a new configuration of a data partition is associated with a different version number. While any suitable scheme may be chosen for implementing version identifiers, in some embodiments, the version identifier for a new configuration of a data partition should also be higher than any other value for the version identifier previously or currently associated with that data partition. Thus, the version identifier for the new configuration should be higher than the version identifier associated with the old configuration. It may also be the intent for the version identifier associated with the new configuration to be higher than any version identifiers associated with any other configurations for that data partition, including version identifiers associated with failed reconfigurations of that data partition. Though, in other embodiments, other mechanisms may be used to ascertain a current version identifier.

At block 460, the primary node may determine a new value for the version identifier associated with the new configuration. In some embodiments, the primary node may select a value higher than the version number associated with the old configuration. The primary node may then send a message containing this proposed value for the new version identifier to nodes in both the new and the old configurations. The primary node may then wait until it has received responses from at least a quorum of nodes in the new configuration and a quorum of nodes in the old configuration indicating that the nodes have accepted the proposed version identifier for the new configuration.

In some embodiments, the quorum associated with the old and new configurations will be the same or may be the same percentage of the nodes in the configuration. However, it is not a requirement that the quorums associated with the old and new configurations be the same or be determined in the same way. In some embodiments, the primary node may wait for a read quorum of nodes in the old configuration to accept, and a write quorum of nodes in the new configuration to accept.

In some embodiments, the reconfiguration process may not proceed (i.e., may be suspended) until a read quorum of nodes in the old configuration and a write quorum of nodes in the new configuration accept the proposed version identifier. Furthermore, in some embodiments, even after a read quorum of nodes in the old configuration and a write quorum of nodes in the new configuration have accepted the new proposed version identifier, the primary node may still wait for a pre-determined time, or other event, to allow slower replicas to respond, and possibly reject the proposed version identifier.

At block 462, the primary node may then check if it has received a response from any node in either the new configuration or the old configuration indicating that the node has rejected the proposal. A node may reject the proposed version identifier for a number of reasons. For example, a node may reject the proposal if it has previously seen a version identifier associated with the data partition that is higher than the proposed version identifier. If so, the rejecting node may send in a rejection response to the primary node the version identifier it has previously seen.

The process may then proceed to block 464, in which the node on which the process is running may determine if it is still the primary node for the new configuration. In some embodiments, for a number of reasons, the primary node for a new configuration may change, which may be done in response to a command from the configuration manager. If the node on which the reconfiguration process is running determines at block 464 that it is no longer the primary node, it may, at block 466, abort the reconfiguration process. The reconfiguration process of FIG. 4C may be done at this point.

Otherwise, if it is determined at block 464 that the node on which the reconfiguration process is running is still the primary, the primary node may restart the reconfiguration process by returning to block 460. It may then repeat the process discussed above, this time selecting a new proposed version identifier for the new configuration that is higher than any previously seen version identifier for the configuration, including version identifiers included in a rejection message sent in response to a previous proposal.

Otherwise, if the primary node determined at block 462 that no nodes rejected the proposal, then, at block 468, the primary node may update data replicated in nodes of the new configuration with the most recent data replicated in the old configuration. This may be done in any suitable way. In some embodiments, this operation may be done in two steps. First, if the primary node does not contain the most recent copy of the replica from the old configuration, then the primary node may update (or “catch-up”) its local replica of the data partition from a node in the old configuration with the most recent data. In some embodiments, the node in the old configuration with the most recent data is selected from among the nodes in the old configuration that accepted the version identifier, as discussed in step 460. Determining that a node contains the latest data may be done in any suitable way. In some embodiments, it may be done by checking an identifier associated with the latest transaction stored in the replica.

Once the primary node has the most recent copy of the replica, in some embodiments, the primary node may then disable user transactions on the data partition, and instruct the other nodes in the new configuration to update their replicas. The primary node may then wait for a write quorum of nodes in the new configuration to be updated.

In some embodiments, updating a replica may involve updating the replica to store at least all the transactions that had been committed by a write quorum of replicas in the old configuration. In some embodiments, replicas on a write quorum of nodes in the new configuration may be updated to store a consistent set of transactions. Updating a replica may thus also include transaction roll-backs, if a particular replica in the new configuration contains a later transaction than that which was committed by a write quorum of nodes in the old configuration. Such a situation could arise if a user transaction was aborted before it was committed, such that the transaction was stored on less than a write quorum of nodes in the old configuration. In some embodiments, the reconfiguration process may not proceed (i.e., may be suspended) until a write quorum of replicas in the new configuration have been updated to store a consistent set of transactions.

Once a write quorum of nodes in the new configuration has been caught up, the reconfiguration process may involve sending a reconfiguration message to the nodes of the old configuration and/or the new configuration, and waiting for a quorum of nodes in the given configuration to accept the message. The reconfiguration message may include, for example, an activation message or a deactivation message. For example, in the example illustrated by FIG. 4C, at block 470, the primary node may then send a deactivation message to the nodes in the old configuration. The deactivation message may comprise the proposed new configuration, which may include identifiers of the nodes in the proposed new configuration. If a node in the old configuration accepts the deactivation message, it may change the state of its current active configuration to being “Deactivated,” and may store the proposed new configuration in suitable computer storage on the node, such as on a hard disk.

Each node in the old configuration may also be configured to send back a response to the primary node indicating whether the node accepts or rejects the deactivation message. The primary node may then wait for a read quorum of responses from nodes in the old configuration indicating that they have accepted the deactivation message.

Next, at block 472, the primary node may determine if a response to the deactivation message from at least one node in the old configuration indicates that the node has rejected the deactivation message. If any node has rejected the deactivation message, the reconfiguration process may proceed to block 464, at which point the reconfiguration process will either restart or abort, as discussed above.

Otherwise, if it was determined at block 472 that no node in the old configuration rejected the deactivation message, once a read quorum of nodes in the old configuration have indicated that they have accepted the reconfiguration message, at block 474, the primary node may begin the activation of the new configuration. This may be done by sending an activation message to all the other nodes in the new configuration. Each of the other nodes in the new configuration may be configured to send a response to the primary node indicating whether the node accepted or rejected the activation message. If a node accepts the activation message, it may update its state to an “activated” state, appropriately. In some embodiments, a node that is both part of the old configuration and the new configuration may have already stored locally (e.g., after accepting the deactivation message) the state of the new configuration as a proposed configuration, in which case it may, at block 474, update the stored state from “proposed” to “activated.” The primary node may then wait for a write quorum of responses from nodes in the new configuration indicating that they have accepted the activation message.

Next, at block 476, the primary node may determine if a response to the activation message from at least one node in the new configuration indicates that the node has rejected the activation message. If any node has rejected the activation message, the reconfiguration process may proceed to block 464, at which point the reconfiguration process will either restart or abort, as discussed above.

However, if a write quorum of nodes in the new configuration accepted the activation message and no node rejected the activation message, the primary node may then re-enable transactions on the configuration, and may also notify the configuration manager, such as the configuration manager 421, of the committed state of the new configuration. The process may also proceed to block 478, at which point the primary node may commit the new configuration. This optional operation may involve sending a configuration commit message containing both the old configuration and the new configuration to all nodes in the network having replicas of the data partition subject to the new configuration. This step may be optional, because in some embodiments, the new configuration may become active and complete as soon as a write quorum of nodes in the new configuration has accepted the activation message. The process of FIG. 4C may be done at this point, as the new configuration may be fully activated and usable.

As used in this application, the terms “component”, “system”, are intended to refer to a computer-related entity, either hardware, a combination of hardware and software, software, or software in execution. For example, a component can be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer. By way of illustration, both an application running on a server and the server can be a component. One or more components can reside within a process and/or thread of execution, and a component can be localized on one computer and/or distributed between two or more computers.

Furthermore, all or portions of the subject innovation can be implemented as a system, method, apparatus, or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware or any combination thereof to control a computer to implement the disclosed innovation. For example, computer readable media can include but are not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips . . . ), optical disks (e.g., compact disk (CD), digital versatile disk (DVD) . . . ), smart cards, and flash memory devices (e.g., card, stick, key drive . . . ). Additionally it should be appreciated that a carrier wave can be employed to carry computer-readable electronic data such as those used in transmitting and receiving electronic mail or in accessing a network such as the Internet or a local area network (LAN). Of course, those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope or spirit of the claimed subject matter.

In order to provide a context for the various aspects of the disclosed subject matter, FIGS. 5 and 6 as well as the following discussion are intended to provide a brief, general description of a suitable environment in which the various aspects of the disclosed subject matter may be implemented. While the subject matter has been described above in the general context of computer-executable instructions of a computer program that runs on a computer and/or computers, those skilled in the art will recognize that the innovation also may be implemented in combination with other program modules. Generally, program modules include routines, programs, components, data structures, and the like, which perform particular tasks and/or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the innovative methods can be practiced with other computer system configurations, including single-processor or multiprocessor computer systems, mini-computing devices, mainframe computers, as well as personal computers, hand-held computing devices (e.g., personal digital assistant (PDA), phone, watch . . . ), microprocessor-based or programmable consumer or industrial electronics, and the like. The illustrated aspects may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. However, some, if not all aspects of the innovation can be practiced on stand-alone computers. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.

With reference to FIG. 5, an exemplary environment 510 for implementing various aspects of the subject innovation is described that includes a computer 512. The computer 512 includes a processing unit 514, a system memory 516, and a system bus 518. The system bus 518 couples system components including, but not limited to, the system memory 516 to the processing unit 514. The processing unit 514 can be any of various available processors. Dual microprocessors and other multiprocessor architectures also can be employed as the processing unit 514.

The system bus 518 can be any of several types of bus structure(s) including the memory bus or memory controller, a peripheral bus or external bus, and/or a local bus using any variety of available bus architectures including, but not limited to, II-bit bus, Industrial Standard Architecture (ISA), Micro-Channel Architecture (MSA), Extended ISA (EISA), Intelligent Drive Electronics (IDE), VESA Local Bus (VLB), Peripheral Component Interconnect (PCI), Universal Serial Bus (USB), Advanced Graphics Port (AGP), Personal Computer Memory Card International Association bus (PCMCIA), and Small Computer Systems Interface (SCSI).

The system memory 516 includes volatile memory 520 and nonvolatile memory 522. The basic input/output system (BIOS), containing the basic routines to transfer information between elements within the computer 512, such as during start-up, is stored in nonvolatile memory 522. For example, nonvolatile memory 522 can include read only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable ROM (EEPROM), or flash memory. Volatile memory 520 includes random access memory (RAM), which acts as external cache memory. By way of illustration and not limitation, RAM is available in many forms such as synchronous RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), Synchlink DRAM (SLDRAM), and direct Rambus RAM (DRRAM).

Computer 512 also includes removable/non-removable, volatile/nonvolatile computer storage media. FIG. 5 illustrates a disk storage 524, wherein such disk storage 524 includes, but is not limited to, devices like a magnetic disk drive, floppy disk drive, tape drive, Jaz drive, Zip drive, LS-60 drive, flash memory card, or memory stick. In addition, disk storage 524 can include storage media separately or in combination with other storage media including, but not limited to, an optical disk drive such as a compact disk ROM device (CD-ROM), CD recordable drive (CD-R Drive), CD rewritable drive (CD-RW Drive) or a digital versatile disk ROM drive (DVD-ROM). To facilitate connection of the disk storage devices 524 to the system bus 518, a removable or non-removable interface is typically used such as interface 526.

It is to be appreciated that FIG. 5 describes software that acts as an intermediary between users and the basic computer resources described in suitable operating environment 510. Such software includes an operating system 528. Operating system 528, which can be stored on disk storage 524, acts to control and allocate resources of the computer system 512. System applications 530 take advantage of the management of resources by operating system 528 through program modules 532 and program data 534 stored either in system memory 516 or on disk storage 524. It is to be appreciated that various components described herein can be implemented with various operating systems or combinations of operating systems.

A user enters commands or information into the computer 512 through input device(s) 536. Input devices 536 include, but are not limited to, a pointing device such as a mouse, trackball, stylus, touch pad, keyboard, microphone, joystick, game pad, satellite dish, scanner, TV tuner card, digital camera, digital video camera, web camera, and the like. These and other input devices connect to the processing unit 514 through the system bus 518 via interface port(s) 538. Interface port(s) 538 include, for example, a serial port, a parallel port, a game port, and a universal serial bus (USB). Output device(s) 540 use some of the same type of ports as input device(s) 536. Thus, for example, a USB port may be used to provide input to computer 512, and to output information from computer 512 to an output device 540. Output adapter 542 is provided to illustrate that there are some output devices 540 like monitors, speakers, and printers, among other output devices 540 that require special adapters. The output adapters 542 include, by way of illustration and not limitation, video and sound cards that provide a means of connection between the output device 540 and the system bus 518. It should be noted that other devices and/or systems of devices provide both input and output capabilities such as remote computer(s) 544.

Computer 512 can operate in a networked environment using logical connections to one or more remote computers, such as remote computer(s) 544. The remote computer(s) 544 can be a personal computer, a server, a router, a network PC, a workstation, a microprocessor based appliance, a peer device or other common network node and the like, and typically includes many or all of the elements described relative to computer 512. For purposes of brevity, only a memory storage device 546 is illustrated with remote computer(s) 544. Remote computer(s) 544 is logically connected to computer 512 through a network interface 548 and then physically connected via communication connection 550. Network interface 548 encompasses communication networks such as local-area networks (LAN) and wide area networks (WAN). LAN technologies include Fiber Distributed Data Interface (FDDI), Copper Distributed Data Interface (CDDI), Ethernet/IEEE 802.3, Token Ring/IEEE 802.5 and the like. WAN technologies include, but are not limited to, point-to-point links, circuit switching networks like Integrated Services Digital Networks (ISDN) and variations thereon, packet switching networks, and Digital Subscriber Lines (DSL).

Communication connection(s) 550 refers to the hardware/software employed to connect the network interface 548 to the bus 518. While communication connection 550 is shown for illustrative clarity inside computer 512, it can also be external to computer 512. The hardware/software necessary for connection to the network interface 548 includes, for exemplary purposes only, internal and external technologies such as, modems including regular telephone grade modems, cable modems and DSL modems, ISDN adapters, and Ethernet cards.

FIG. 6 is a schematic block diagram of a sample-computing environment 600 that can be employed for implementing nodes as part of a federation, in accordance with an aspect of the subject innovation. The system 600 includes one or more client(s) 610. The client(s) 610 can be hardware and/or software (e.g., threads, processes, computing devices). The system 600 also includes one or more server(s) 630. The server(s) 630 can also be hardware and/or software (e.g., threads, processes, computing devices). The servers 630 can house threads to perform transformations by employing the components described herein, for example. One possible communication between a client 610 and a server 630 may be in the form of a data packet adapted to be transmitted between two or more computer processes. The system 600 includes a communication framework 650 that can be employed to facilitate communications between the client(s) 610 and the server(s) 630. The client(s) 610 are operatively connected to one or more client data store(s) 660 that can be employed to store information local to the client(s) 610. Similarly, the server(s) 630 are operatively connected to one or more server data store(s) 640 that can be employed to store information local to the servers 630.

Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. As an example of a possible variation, in an exemplary embodiment described above, a quorum of nodes was selected to be a majority of the nodes. Other implementations are possible, with the quorum being either greater or less than a majority of the nodes. Moreover, the quorum may change over time for a configuration as nodes fail or go off-line.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

The above-described embodiments of the present invention can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in any of a number of forms, such as a rack-mounted computer, a desktop computer, a laptop computer, or a tablet computer. Additionally, a computer may be embedded in a device not generally regarded as a computer but with suitable processing capabilities, including a Personal Digital Assistant (PDA), a smart phone or any other suitable portable or fixed electronic device.

Also, a computer may have one or more input and output devices. These devices can be used, among other things, to present a user interface. Examples of output devices that can be used to provide a user interface include printers or display screens for visual presentation of output and speakers or other sound generating devices for audible presentation of output. Examples of input devices that can be used for a user interface include keyboards, and pointing devices, such as mice, touch pads, and digitizing tablets. As another example, a computer may receive input information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in any suitable form, including as a local area network or a wide area network, such as an enterprise network or the Internet. Such networks may be based on any suitable technology and may operate according to any suitable protocol and may include wireless networks, wired networks or fiber optic networks.

Also, the various methods or processes outlined herein may be coded as software that is executable on one or more processors that employ any one of a variety of operating systems or platforms. Additionally, such software may be written using any of a number of suitable programming languages and/or programming or scripting tools, and also may be compiled as executable machine language code or intermediate code that is executed on a framework or virtual machine.

In this respect, the invention may be embodied as a computer readable medium (or multiple computer readable media) (e.g., a computer memory, one or more floppy discs, compact discs, optical discs, magnetic tapes, flash memories, circuit configurations in Field Programmable Gate Arrays or other semiconductor devices, or other tangible computer storage medium) encoded with one or more programs that, when executed on one or more computers or other processors, perform methods that implement the various embodiments of the invention discussed above. The computer readable medium or media can be transportable, such that the program or programs stored thereon can be loaded onto one or more different computers or other processors to implement various aspects of the present invention as discussed above.

The terms “program” or “software” are used herein in a generic sense to refer to any type of computer code or set of computer-executable instructions that can be employed to program a computer or other processor to implement various aspects of the present invention as discussed above. Additionally, it should be appreciated that according to one aspect of this embodiment, one or more computer programs that when executed perform methods of the present invention need not reside on a single computer or processor, but may be distributed in a modular fashion amongst a number of different computers or processors to implement various aspects of the present invention.

Computer-executable instructions may be in many forms, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically the functionality of the program modules may be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in any suitable form. For simplicity of illustration, data structures may be shown to have fields that are related through location in the data structure. Such relationships may likewise be achieved by assigning storage for the fields with locations in a computer-readable medium that conveys relationship between the fields. However, any suitable mechanism may be used to establish a relationship between information in fields of a data structure, including through the use of pointers, tags or other mechanisms that establish relationship between data elements.

Various aspects of the present invention may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 

What is claimed is:
 1. A computing device, comprising: a memory and a processor that are respectively configured to store and execute instructions, including instructions for causing the computing device to perform operations for reconfiguring at least a portion of a distributed database replicated on the first set of nodes in a first configuration to a second set of nodes in a new configuration, the at least the portion of the distributed database storing a plurality of transactions, the reconfiguration operations including: determining a version identifier associated with the new configuration; updating transactions stored on the new configuration with transactions stored on the first configuration; deactivating the first configuration; activating the new configuration; and committing the new configuration, wherein proceeding with each of the reconfiguration operations only occurs if a number of responses containing an indication of acceptance of the particular reconfiguration operation is at least a quorum value associated with nodes on which the particular operation is to be performed.
 2. The computing device of claim 1, wherein: each configuration of the first configuration and the new configuration is associated with at least one quorum value specifying a number of nodes in the configuration; and determining a version identifier associated with the new configuration comprises: selecting a version identifier associated with the new configuration, wherein the selected version identifier associated with the new configuration has a greater value than the value of the version identifier associated with the first configuration; sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the version identifier associated with the new configuration; receiving a response to the reconfiguration proposal from each node in at least a subset of the first configuration and from each node in at least a subset of the new configuration, wherein each response received from a node contains an indication of an acceptance or a rejection of the reconfiguration proposal; determining whether a received response to the reconfiguration proposal contains an indication of a rejection of the reconfiguration proposal; and if it is determined that a received response contains an indication of a rejection of the reconfiguration proposal: selecting a new version identifier associated with the new configuration, wherein the new version identifier associated with the new configuration has a greater value than a version identifier included in the received response containing an indication of a rejection of the reconfiguration proposal; and sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the new version identifier associated with the new configuration; determining whether a first number of received responses from the first configuration containing an indication of an acceptance of the proposal message is at least a first quorum value associated with the first configuration; determining whether a second number of received responses from the new configuration containing an indication of an acceptance of the proposal message is at least a first quorum value associated with the new configuration; and if it is determined that the first number is at least a first quorum value associated with the first configuration and that the second number is at least a first quorum value associated with the new configuration: performing the acts of updating, deactivating, activating, and committing.
 3. The computing device of claim 2, wherein updating transactions stored on the new configuration with transactions stored on the first configuration comprises: after it is determined that the first number is at least the first quorum value associated with the first configuration and that the second number is at least the first quorum value associated with the new configuration: instructing each node in the new configuration to update a replica of the at least a portion of the distributed database stored on the node, wherein the updated replica contains each transaction that has been stored as being committed on at least a subset of the first configuration, the size of the subset of the first configuration being a second quorum value associated with the first configuration; determining whether a number of nodes in the new configuration that has updated the replica is at least the first quorum value associated with the new configuration; and if it is determined that the number of the nodes in the new configuration that has updated the replica is at least the first quorum value for the new configuration, performing the acts of deactivating, activating, and committing.
 4. The computing device of claim 3, wherein deactivating the first configuration comprises: sending a deactivation message to each of a plurality of nodes in the first configuration; receiving a response to the deactivation message from each node in at least a subset of the first configuration, wherein each response received from a node in the first configuration contains an indication of an acceptance or a rejection of the deactivation message; determining whether a number of received responses containing an indication of an acceptance of the deactivation message is at least the first quorum value associated with the first configuration; and if the number of received responses containing an indication of an acceptance of the deactivation message is at least the first quorum value associated with the first configuration, performing the acts of activating and committing.
 5. The computing device of claim 4, wherein: activating the first configuration comprises: sending an activation message to each of a plurality of nodes in the new configuration; receiving a response to the activation message from each node in at least a subset of the new configuration, wherein each response received from a node in the new configuration contains an indication of an acceptance or a rejection of the activation message; determining whether a number of received responses containing an indication of an acceptance of the activation message is at least the first quorum value associated with the new configuration; and if the number of received responses containing an indication of an acceptance of the activation message is at least the first quorum value associated with the new configuration, performing the act of committing; and committing the first configuration comprises: sending a configuration commit message to the configuration manager, each of a plurality of nodes in the first configuration and each of a plurality of nodes in the new configuration, the configuration commit message comprising identifiers for the nodes in the first configuration and identifiers for the nodes in the new configuration.
 6. The computing device of claim 1, wherein: each configuration of the first configuration and the new configuration is associated with at least one quorum value specifying a number of nodes in the configuration; and determining a version identifier associated with the new configuration comprises: selecting a version identifier associated with the new configuration, wherein the selected version identifier associated with the new configuration has a greater value than the value of the version identifier associated with the first configuration; sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the version identifier associated with the new configuration; receiving a response to the reconfiguration proposal from each node in at least a subset of the first configuration and from each node in at least a subset of the new configuration, wherein each response received from a node contains an indication of an acceptance or a rejection of the reconfiguration proposal; determining whether a received response to the reconfiguration proposal contains an indication of a rejection of the reconfiguration proposal; and if it is determined that a received response contains an indication of a rejection of the reconfiguration proposal, restarting the method of reconfiguring.
 7. The computing device of claim 1, wherein deactivating the first configuration comprises: sending a deactivation message to each of a plurality of nodes in the first configuration; receiving a response to the deactivation message from each node in at least a subset of the first configuration, wherein each response received from a node in the first configuration contains an indication of an acceptance or a rejection of the deactivation message; after receiving a response to the deactivation message from each node in at least a subset of the first configuration, determining whether a received response to the deactivation message contains an indication of a rejection of the deactivation message; and if it is determined that a received response to the deactivation message contains an indication of a rejection of the deactivation message, restarting the method of reconfiguring.
 8. A computer-readable storage medium, comprising at least one of a memory, disk, disc, or device, that is encoded with computer-executable instructions that, when executed on at least one processor in a computer in a network, implement a method of replicating at least a portion of a distributed database on a first set of nodes in a first configuration to a second set of nodes in a new configuration, the at least the portion of the distributed database storing a plurality of transactions, the method comprising reconfiguration operations including: determining a version identifier associated with the new configuration; updating transactions stored on the new configuration with transactions stored on the first configuration; deactivating the first configuration; activating the new configuration; and committing the new configuration, wherein proceeding with each of the reconfiguration operations only occurs if a number of responses containing an indication of acceptance of the particular reconfiguration operation is at least a quorum value associated with nodes on which the particular operation is proposed to be performed on.
 9. The computer-readable storage medium of claim 8, wherein: each configuration of the first configuration and the new configuration is associated with at least one quorum value specifying a number of nodes in the configuration; and determining a version identifier associated with the new configuration comprises: selecting a version identifier associated with the new configuration, wherein the selected version identifier associated with the new configuration has a greater value than the value of the version identifier associated with the first configuration; sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the version identifier associated with the new configuration; receiving a response to the reconfiguration proposal from each node in at least a subset of the first configuration and from each node in at least a subset of the new configuration, wherein each response received from a node contains an indication of an acceptance or a rejection of the reconfiguration proposal; determining whether a received response to the reconfiguration proposal contains an indication of a rejection of the reconfiguration proposal; and if it is determined that a received response contains an indication of a rejection of the reconfiguration proposal: selecting a new version identifier associated with the new configuration, wherein the new version identifier associated with the new configuration has a greater value than a version identifier included in the received response containing an indication of a rejection of the reconfiguration proposal; and sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the new version identifier associated with the new configuration; determining whether a first number of received responses from the first configuration containing an indication of an acceptance of the proposal message is at least a first quorum value associated with the first configuration; determining whether a second number of received responses from the new configuration containing an indication of an acceptance of the proposal message is at least a first quorum value associated with the new configuration; and if it is determined that the first number is at least a first quorum value associated with the first configuration and that the second number is at least a first quorum value associated with the new configuration: performing the acts of updating, deactivating, activating, and committing.
 10. The computer-readable storage medium of claim 9, wherein updating transactions stored on the new configuration with transactions stored on the first configuration comprises: after it is determined that the first number is at least the first quorum value associated with the first configuration and that the second number is at least the first quorum value associated with the new configuration: instructing each node in the new configuration to update a replica of the at least a portion of the distributed database stored on the node, wherein the updated replica contains each transaction that has been stored as being committed on at least a subset of the first configuration, the size of the subset of the first configuration being a second quorum value associated with the first configuration; determining whether a number of nodes in the new configuration that has updated the replica is at least the first quorum value associated with the new configuration; and if it is determined that the number of the nodes in the new configuration that has updated the replica is at least the first quorum value for the new configuration, performing the acts of deactivating, activating, and committing.
 11. The computer-readable storage medium of claim 10, wherein deactivating the first configuration comprises: sending a deactivation message to each of a plurality of nodes in the first configuration; receiving a response to the deactivation message from each node in at least a subset of the first configuration, wherein each response received from a node in the first configuration contains an indication of an acceptance or a rejection of the deactivation message; determining whether a number of received responses containing an indication of an acceptance of the deactivation message is at least the first quorum value associated with the first configuration; and if the number of received responses containing an indication of an acceptance of the deactivation message is at least the first quorum value associated with the first configuration, performing the acts of activating and committing.
 12. The computer-readable storage medium of claim 11, wherein: activating the first configuration comprises: sending an activation message to each of a plurality of nodes in the new configuration; receiving a response to the activation message from each node in at least a subset of the new configuration, wherein each response received from a node in the new configuration contains an indication of an acceptance or a rejection of the activation message; determining whether a number of received responses containing an indication of an acceptance of the activation message is at least the first quorum value associated with the new configuration; and if the number of received responses containing an indication of an acceptance of the activation message is at least the first quorum value associated with the new configuration, performing the act of committing; and committing the first configuration comprises: sending a configuration commit message to the configuration manager, each of a plurality of nodes in the first configuration and each of a plurality of nodes in the new configuration, the configuration commit message comprising identifiers for the nodes in the first configuration and identifiers for the nodes in the new configuration.
 13. The computer-readable storage medium of claim 8, wherein: each configuration of the first configuration and the new configuration is associated with at least one quorum value specifying a number of nodes in the configuration; and determining a version identifier associated with the new configuration comprises: selecting a version identifier associated with the new configuration, wherein the selected version identifier associated with the new configuration has a greater value than the value of the version identifier associated with the first configuration; sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the version identifier associated with the new configuration; receiving a response to the reconfiguration proposal from each node in at least a subset of the first configuration and from each node in at least a subset of the new configuration, wherein each response received from a node contains an indication of an acceptance or a rejection of the reconfiguration proposal; determining whether a received response to the reconfiguration proposal contains an indication of a rejection of the reconfiguration proposal; and if it is determined that a received response contains an indication of a rejection of the reconfiguration proposal, restarting the method of reconfiguring.
 14. A method of reconfiguring at least a portion of a distributed database replicated on a first set of nodes in a first configuration to a second set of nodes in a new configuration, the at least the portion of the distributed database storing a plurality of transactions, the method comprising reconfiguration operations including: determining a version identifier associated with the new configuration; updating transactions stored on the new configuration with transactions stored on the first configuration; deactivating the first configuration; activating the new configuration; and committing the new configuration, wherein proceeding with each of the reconfiguration operations only occurs if a number of responses containing an indication of acceptance of the particular reconfiguration operation is at least a quorum value associated with nodes on which the particular operation is to be performed.
 15. The method of claim 14, wherein: each configuration of the first configuration and the new configuration is associated with at least one quorum value specifying a number of nodes in the configuration; and determining a version identifier associated with the new configuration comprises: selecting a version identifier associated with the new configuration, wherein the selected version identifier associated with the new configuration has a greater value than the value of the version identifier associated with the first configuration; sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the version identifier associated with the new configuration; receiving a response to the reconfiguration proposal from each node in at least a subset of the first configuration and from each node in at least a subset of the new configuration, wherein each response received from a node contains an indication of an acceptance or a rejection of the reconfiguration proposal; determining whether a received response to the reconfiguration proposal contains an indication of a rejection of the reconfiguration proposal; and if it is determined that a received response contains an indication of a rejection of the reconfiguration proposal: selecting a new version identifier associated with the new configuration, wherein the new version identifier associated with the new configuration has a greater value than a version identifier included in the received response containing an indication of a rejection of the reconfiguration proposal; and sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the new version identifier associated with the new configuration; determining whether a first number of received responses from the first configuration containing an indication of an acceptance of the proposal message is at least a first quorum value associated with the first configuration; determining whether a second number of received responses from the new configuration containing an indication of an acceptance of the proposal message is at least a first quorum value associated with the new configuration; and if it is determined that the first number is at least a first quorum value associated with the first configuration and that the second number is at least a first quorum value associated with the new configuration: performing the acts of updating, deactivating, activating, and committing.
 16. The method of claim 15, wherein updating transactions stored on the new configuration with transactions stored on the first configuration comprises: after it is determined that the first number is at least the first quorum value associated with the first configuration and that the second number is at least the first quorum value associated with the new configuration: instructing each node in the new configuration to update a replica of the at least a portion of the distributed database stored on the node, wherein the updated replica contains each transaction that has been stored as being committed on at least a subset of the first configuration, the size of the subset of the first configuration being a second quorum value associated with the first configuration; determining whether a number of nodes in the new configuration that has updated the replica is at least the first quorum value associated with the new configuration; and if it is determined that the number of the nodes in the new configuration that has updated the replica is at least the first quorum value for the new configuration, performing the acts of deactivating, activating, and committing.
 17. The method of claim 16, wherein deactivating the first configuration comprises: sending a deactivation message to each of a plurality of nodes in the first configuration; receiving a response to the deactivation message from each node in at least a subset of the first configuration, wherein each response received from a node in the first configuration contains an indication of an acceptance or a rejection of the deactivation message; determining whether a number of received responses containing an indication of an acceptance of the deactivation message is at least the first quorum value associated with the first configuration; and if the number of received responses containing an indication of an acceptance of the deactivation message is at least the first quorum value associated with the first configuration, performing the acts of activating and committing.
 18. The method of claim 17, wherein: activating the first configuration comprises: sending an activation message to each of a plurality of nodes in the new configuration; receiving a response to the activation message from each node in at least a subset of the new configuration, wherein each response received from a node in the new configuration contains an indication of an acceptance or a rejection of the activation message; determining whether a number of received responses containing an indication of an acceptance of the activation message is at least the first quorum value associated with the new configuration; and if the number of received responses containing an indication of an acceptance of the activation message is at least the first quorum value associated with the new configuration, performing the act of committing; and committing the first configuration comprises: sending a configuration commit message to the configuration manager, each of a plurality of nodes in the first configuration and each of a plurality of nodes in the new configuration, the configuration commit message comprising identifiers for the nodes in the first configuration and identifiers for the nodes in the new configuration.
 19. The method of claim 14, wherein: each configuration of the first configuration and the new configuration is associated with at least one quorum value specifying a number of nodes in the configuration; and determining a version identifier associated with the new configuration comprises: selecting a version identifier associated with the new configuration, wherein the selected version identifier associated with the new configuration has a greater value than the value of the version identifier associated with the first configuration; sending to each of a plurality of nodes in the first configuration and to each of a plurality of nodes in the new configuration a reconfiguration proposal comprising the version identifier associated with the new configuration; receiving a response to the reconfiguration proposal from each node in at least a subset of the first configuration and from each node in at least a subset of the new configuration, wherein each response received from a node contains an indication of an acceptance or a rejection of the reconfiguration proposal; determining whether a received response to the reconfiguration proposal contains an indication of a rejection of the reconfiguration proposal; and if it is determined that a received response contains an indication of a rejection of the reconfiguration proposal, restarting the method of reconfiguring.
 20. The method of claim 14, wherein deactivating the first configuration comprises: sending a deactivation message to each of a plurality of nodes in the first configuration; receiving a response to the deactivation message from each node in at least a subset of the first configuration, wherein each response received from a node in the first configuration contains an indication of an acceptance or a rejection of the deactivation message; after receiving a response to the deactivation message from each node in at least a subset of the first configuration, determining whether a received response to the deactivation message contains an indication of a rejection of the deactivation message; and if it is determined that a received response to the deactivation message contains an indication of a rejection of the deactivation message, restarting the method of reconfiguring. 